Exhaust gas purifying system for internal combustion engine

ABSTRACT

An exhaust gas purifying system for an internal combustion engine has a relatively simple construction and is capable of accurately calculating the amount of SOx deposited on a NOx catalyst. 
     A CO2 sensor having a detection electrode containing BaCO3 is disposed in an intake pipe at a location downstream of a confluent portion. A catalyst SOx deposition amount is calculated according to an EGR rate and a sensor deposition amount, based on the fact that the relationship between the sensor deposition amount detected by the CO2 sensor and the catalyst SOx deposition amount is determined according to the EGR rate.

TECHNICAL FIELD

The present invention relates to an exhaust gas purifying system for an internal combustion engine, for purifying exhaust gases discharged from the engine to an exhaust passage, and more particularly to an exhaust gas purifying system for calculating the amount of SOx deposited on a NOx catalyst.

BACKGROUND ART

Conventionally, as an exhaust gas purifying system of this kind, there has been known, for example, one disclosed in Patent Literature 1. A NOx catalyst for purifying NOx in exhaust gases is disposed in an exhaust pipe of the engine. In the NOx catalyst, a plurality of bed temperature sensors for detecting the temperatures of catalyst beds of the NOx catalyst (catalyst bed temperatures) are arranged at equally-spaced intervals along the direction of flow of exhaust gases. Further, in the exhaust gas purifying system, under a reducing atmosphere, there are executed not only NOx reduction control for eliminating NOx deposited on the NOx catalyst, but also PM elimination control for eliminating particulate matter (PM) from the NOx catalyst and SOx elimination control for eliminating SOx from the NOx catalyst are executed.

In this exhaust gas purifying system, during execution of the PM elimination control, when the temperature of exhaust gases enters a light-off temperature region (temperature region where the NOx catalyst is activated to cause the catalyst bed temperatures to sharply raise), the amount of rise of each catalyst bed temperature caused by supply of reducing agents to the NOx catalyst (hereinafter referred to as the “catalyst temperature rise amount”) is calculated based on a detection value from each bed temperature sensor. Then, when any of the catalyst temperature rise amounts is smaller than a predetermined reference value, judging that the amount of SOx deposited on the NOx catalyst is too large, the SOx deposition amount is calculated based on the catalyst temperature rise amounts, and timing for executing the SOx elimination control and the like are determined based on the calculated SOx deposition amount.

Further, as another conventional exhaust gas purifying system, one disclosed in Patent Literature 2 is known. This exhaust gas purifying system is provided with a NOx catalyst in the exhaust pipe, and a λ sensor for detecting an excess air ratio λ in exhaust gases is disposed in the exhaust pipe at a location downstream of the NOx catalyst. Further, in the exhaust gas purifying system, under a reducing atmosphere, NOx reduction control for reducing NOx trapped in the NOx catalyst and SOx elimination control for eliminating SOx deposited on the NOx catalyst are executed.

In the exhaust gas purifying system, the air-fuel ratio of exhaust gases is calculated using the excess ratio λ detected by the λ sensor, and a time period after the start of the SOx elimination control until the air-fuel ratio of exhaust gases switches from a lean value to a rich value with respect to the stoichiometric air-fuel ratio is measured. Then, the SOx deposition amount at the start of the SOx elimination control is calculated according to the measured time period.

CITATION LIST

-   [PTL 1] Japanese Laid-Open Patent Publication (Kokai) No.     2009-138525 -   [PTL 2] European Patent Application Publication No. 1489414

SUMMARY OF INVENTION Technical Problem

As described above, in the exhaust gas purifying system proposed in Patent Literature 1, only during the PM elimination control, it is determined whether or not the SOx deposition amount has become too large. Therefore, the SOx deposition amount is not calculated during a time period after termination of the PM elimination control until the following start of the PM elimination control, so that when the time period is long, there is a fear that the engine is operated in a state where the SOx deposition amount is too large.

Further, in general, the λ sensor used in the exhaust gas purifying system proposed in Patent Literature 2 has a low sensitivity to a change in the concentration of oxygen in a region where the air-fuel ratio of a mixture is in a lean range, and hence the accuracy of the SOx deposition amount calculated using detection values from the λ sensor is low.

The present invention has been made to provide a solution to the above-described problem, and an object thereof is to provide an exhaust gas purifying system for an internal combustion engine, which has a relatively simple construction and is capable of accurately calculating the amount of SOx deposited on a NOx catalyst, thereby making it possible to improve the fuel economy of the engine and reduce exhaust emissions therefrom.

Solution to Problem

To attain the above object, the invention as claimed in claim 1 provides an exhaust gas purifying system 1 for an internal combustion engine 3, for purifying exhaust gases discharged from the engine 3 to an exhaust passage (exhaust pipe 5 in the embodiment (the same applies hereinafter in this section)), comprising a NOx catalyst 7 disposed in the exhaust passage, for purifying NOx in exhaust gases, an EGR passage (EGR pipe 6 a) that branches from the exhaust passage on an upstream side of the NOx catalyst 7 and joins an intake passage (intake pipe 4), for recirculating part of exhaust gases discharged into the exhaust passage into the intake passage, a CO2 sensor 22 that is disposed in the intake passage at a location downstream of a confluent portion 4 a where the FOR passage joins the intake passage, and includes a detection electrode 22 a containing BaCO3, for detecting a CO2 concentration in intake air sucked into the engine 3, EGR rate-calculating means (ECU 2, step 1 in FIG. 4) for calculating a recirculation rate of exhaust gases recirculated via the EGR passage as an EGR rate rEGR, and SOx deposition amount-calculating means (ECU 2, steps 38, 39 in FIG. 8) for calculating an amount (catalyst SOx deposition amount SOxLNT) of SOx deposited on the NOx catalyst 7 as a SOx deposition amount, according to the calculated EGR rate rEGR and a detection value (sensor SOx deposition amount SOxSNS) detected by the CO2 sensor 22.

According to this exhaust gas purifying system, NOx in exhaust gases discharged from the engine into the exhaust passage is trapped in the NOx catalyst. Further, through EGR operation, part of exhaust gases (hereinafter referred to as the “EGR gas”) is recirculated into the intake passage via the EGR passage, and after being joined to fresh air introduced in the intake passage, is sucked into the engine as intake air. Further, the CO2 sensor is disposed in the intake passage at a location downstream of the confluent portion where the EGR gas flows into the intake passage.

The detection electrode of this CO2 sensor contains BaCO3 (barium carbonate). Therefore, when SO2 in the EGR gas touches the detection electrode, the SO2 is converted to SO3 by oxidation, and the SO3 is deposited on the detection electrode. When the SO3 is deposited on the detection electrode, a detection signal from the CO2 sensor is changed according to the amount of the deposited SO3 (sulfur oxides such as SO, SO2 and SO3 are hereinafter referred to collectively as “SOx”). Thus, the detection value detected by the CO2 sensor represents not only the concentration of CO2 but also the amount of SOx deposited on the CO2 sensor. Further, normally, the CO2 sensor is disposed in an exhaust gas purifying system provided with the EGR passage, and by using the CO2 sensor of the type containing BaCO3 in the detection electrode, it is possible to detect both the concentration of CO2 in the intake air and the amount of SOx deposited on the CO2 sensor.

Further, the exhaust gas purifying system calculates the recirculation rate of the EGR gas recirculated via the EGR passage (the ratio of the amount of the EGR gas to the whole amount of exhaust gas) as an EGR rate, and calculates the amount of SOx deposited on the NOx catalyst according to the calculated EGR rate and the detection value detected by the CO2 sensor. As described above, SOx in exhaust gases recirculated into the intake passage is deposited on the detection electrode of the CO2 sensor. On the other hand, SOx in exhaust gases having flowed into the NOx catalyst is deposited on the NOx catalyst. Further, the ratio between the amount of SOx recirculated into the intake passage and the amount of SOx flowing into the NOx catalyst is determined according to the EGR rate, and according to these SOx amounts, the amount of SOx deposited on the detection electrode of the CO2 sensor and the amour of SOx. deposited on the NOx catalyst are determined.

Therefore, by using the above-described relationship, it is possible to accurately calculate the amount of SOx deposited on the NOx catalyst according to the FOR rate and the amount of SOx deposited on the CO2 sensor. Further, while supplying a just enough amount of reducing agents according to the thus calculated amount of SOx deposited on the NOx catalyst, it is possible to perform the SOx elimination control for eliminating SOx deposited on the NOx catalyst therefrom. This makes it possible to improve the fuel economy of the engine and reduce exhaust emissions therefrom.

The invention as claimed in claim 2 is the exhaust gas purifying system 1 as claimed in claim 1, further comprising average value-calculating means (ECU 2, step 7 in FIG. 4) for calculating an average value (average EGR rate rEGRavrg) of the EGR rate rEGR over a predetermined time period (predetermined distance Lref), and wherein the SOx deposition amount-calculating means calculates the SOx deposition amount using the calculated average value of the EGR rate rEGR (steps 38, 39 in FIG. 8).

With this configuration, the average value of the EGR rate over the predetermined time period is calculated, and the SOx deposition amount during the predetermined time period is calculated according to the average value of the EGR rate and the detection value detected by the CO2 sensor. For example, when the EGR operation is not performed e.g. during a low-speed operation of the engine, the EGR rate is equal to 0, so that it is impossible to calculate the SOx deposition amount according to the EGR rate assumed at the time. Therefore, by calculating the average value of the EGR rate over the predetermined time period, and using the average value for calculation of the SOx deposition amount, it is possible to calculate the SOx deposition amount even when the EGR operation is not being performed at the time point.

The invention as claimed in claim 3 is the exhaust gas purifying system 1 as claimed in claim 1 or 2, further comprising atmospheric CO2 concentration-obtaining means (ECU 2, step 12 in FIG. 5) for obtaining an atmospheric CO2 concentration, error-calculating means (ECU 2, step 13 in FIG. 5) for calculating an error of the detection value detected by the CO2 sensor 22, the error being caused by a deposition of SOx on the detection electrode 22 a of the CO2 sensor 22, according to the CO2 concentration detected by the CO2 sensor 22 when no exhaust gases are being recirculated via the EGR passage, and the obtained atmospheric CO2 concentration, and correction means (ECU 2, step 22 in FIG. 6) for correcting the detection value detected by the CO2 sensor 22 according to the calculated error.

With this configuration, an error of the detection value detected by the CO2 sensor, caused by the deposition of SOx on the detection electrode of the CO2 sensor, is calculated according to the CO2 concentration that is detected by the CO2 sensor when the EGR operation is not being performed and only fresh air is flowing through the intake passage, and the obtained atmospheric CO2 concentration. The detection value detected by the CO2 sensor is corrected according to the calculated error.

In the state where the FOR operation is not being performed, when no SOx is deposited on the detection electrode of the CO2 sensor, the CO2 concentration detected by the CO2 sensor and the atmospheric CO2 concentration become approximately equal to each other. In contrast, when SOx is deposited on the detection electrode, BaCO3 contained in the detection electrode reacts with sulfur, which causes an error between the detected CO2 concentration and the atmospheric CO2 concentration. The magnitude of the error becomes larger as the amount of SOx deposited on the CO2 sensor becomes larger. As described above, the detection error of the CO2 concentration excellently reflects the amount of SOx deposited on the CO2 sensor. This makes it possible to properly correct the detection value detected by the CO2 sensor based on the detection error of the CO2 concentration, so that even when SOx is deposited on the detection electrode of the CO2 sensor, it is possible, while compensating for the adverse influence of the SOx, to accurately calculate the amount of SOx deposited on the NOx catalyst.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A schematic view of an internal combustion engine to which an exhaust gas purifying system according to the present embodiment is applied.

FIG. 2 A block diagram of the exhaust gas purifying system.

FIG. 3 A schematic cross-sectional view of a CO2 sensor.

FIG. 4 A flowchart showing a process for calculating an average EGR ratio.

FIG. 5 A flowchart showing a process for calculating shift voltage of a CO2 sensor.

FIG. 6 A flowchart showing a process for calculating a sensor SOx deposition amount.

FIG. 7 A map for calculating a sensor SOx deposition amount.

FIG. 8 A flowchart showing a process for calculating a catalyst SOx deposition amount.

FIG. 9 A flowchart showing a SOx elimination control process.

FIG. 10 A timing chart showing an example of an operation obtained by the SOx elimination control process.

DESCRIPTION OF EMBODIMENTS

The present invention will now be described in detail with reference to the drawings showing a preferred embodiment thereof. FIG. 1 schematically shows an exhaust gas purifying system 1 according to the present invention, and an internal combustion engine 3 to which is applied the exhaust gas purifying system 1. The internal combustion engine (hereinafter simply referred to as “the engine”) 3 is a lean burn gasoline engine that has e.g. four cylinders, and is installed on a vehicle V.

The engine 3 is provided with an EGR device 6 that includes an EGR pipe 6 a and an EGR control valve 6 b. One end of the EGR pipe 6 a is connected to an exhaust pipe 5 at a branch portion 5 a, and the other end thereof is connected to an intake pipe 4 at a confluent portion 4 a. During EGR operation, part exhaust gases exhausted from the engine 3 is recirculated into the intake pipe 4 via the EGR pipe 6 a as EGR gas, whereby combustion temperature in the engine 3 is lowered, resulting in a reduced amount of NOx contained in the exhaust gases.

The EGR control valve 6 b is implemented by a linear solenoid valve inserted into the EGR pipe 6 a. The amount of EGR gas is controlled by controlling the duty ratio (EGR duty ratio) EGRduty of electric current supplied to the EGR control valve 6 b using an ECU 2 to thereby linearly control the lift amount of the EGR control valve 6 b. Specifically, as the EGR duty ratio EGRduty is larger, the lift amount of the EGR control valve 6 b becomes larger, i.e. the EGR gas amount becomes larger. Further, when EGRduty=0 holds, the EGR control valve 6 b is fully closed, whereby the EGR operation is stopped, so that the EGR gas amount and an EGR rate rEGR indicative of the recirculation rate of exhaust gases recirculated via the EGR pipe 6 a (ratio of the EGR gas amount to the whole amount of exhaust gas) become equal to 0.

Further, the intake pipe 4 is provided with a throttle valve mechanism 8 at a location upstream of the confluent portion 4 a. The throttle valve mechanism 8 includes a throttle valve 8 a and a TH actuator 8 b for actuating the throttle valve 8 a. The opening degree of the throttle valve 8 a is controlled by driving the TH actuator 8 b by a control signal from the ECU 2, whereby the amount of air (fresh air) sucked into the engine 3 is controlled.

Further, the intake pipe 4 is provided with an air flow sensor 21 at a location upstream of the throttle valve 8 a. Further, the intake pipe 4 is provided with a CO2 sensor 22 and an intake pressure sensor 23 at respective locations downstream of the confluent portion 4 a, from upstream to downstream in the mentioned order. The air flow sensor 21 detects the amount of air sucked into the engine 3 (hereinafter referred to as the “air amount”) Mair, and the intake pressure sensor 23 detects an intake pressure PB on the downstream side of the throttle valve 8 a as absolute pressure. Detection signals indicative of the sensed air amount Mair and the sensed pressure PB are delivered to the ECU 2. Furthermore, delivered to the ECU 2 are a detection signal indicative of atmospheric pressure PA from an atmospheric pressure sensor 24, and a detection signal indicative of a vehicle speed VP, which is the speed of the vehicle V, from a vehicle speed sensor 25.

As shown in FIG. 3, the CO2 sensor 22 includes an electrolyte 22 c, and a detection electrode 22 a and a counter electrode 22 b that are arranged on the upper surface of the electrolyte 22 c. The detection electrode 22 a contains BaCO3, and the electrolyte 22 c is formed of a NASICON (sodium super ion conductive). Further, a heater 22 d for heating the detection electrode 22 a is provided on the lower surface of the electrolyte 22 c.

When the detection electrode 22 a is exposed to intake air, CO2 contained in the intake air is diffused in the periphery of the detection electrode 22 a to change a dissociation equilibrium between the BaCO3 and the CO2. In accordance therewith, metal ion activity within the electrolyte 22 c in the vicinity of the detection electrode 22 a is changed. As a consequence, there occurs a potential difference between the detection electrode 22 a and the counter electrode 22 b, and an output voltage Usens dependent on the potential difference is output from the CO2 sensor 22 to the ECU 2. The ECU 2 calculates a CO2 concentration C(CO2)in in the intake air according to the output voltage Usens and the intake pressure PB.

Further, when EGR gas is being recirculated into the intake pipe 4 by the EGR operation, SOx in the EGR gas is deposited on the detection electrode 22 a. When SOx is deposited on the detection electrode 22 a, the potential difference between the detection electrode 22 a and the counter electrode 22 b is changed according to the amount of deposited SOx. The ECU 2 calculates the amount of SOx deposited on the detection electrode 22 a according to the output voltage Usens from the CO2 sensor 22. The method of calculating the SOx deposition amount will be described hereinafter.

On the other hand, the exhaust pipe 5 is provided with an air-fuel ratio sensor 26 at a location downstream of the branch portion 5 a, and a NOx catalyst 7 at a location further downstream of the same. The air-fuel ratio sensor 26 linearly detects an air-fuel ratio in exhaust gases (excess air ratio) λ over a wide range of the air-fuel ratio from a rich region to a lean region, to deliver a signal indicative of the sensed excess air ratio λ to the ECU 2. Further, the NOx catalyst 7 traps NOx contained in exhaust gases under an oxidizing atmosphere in which the concentration of oxygen is high, and reduces the trapped NOx, under a reducing atmosphere in which large amounts of reducing agents are contained in the exhaust gases, to thereby purify exhaust gases.

The ECU 2 is implemented by a microcomputer comprising an I/O interface, a CPU, a RAM and a ROM (none of which are shown). Further, the ECU 2 determines operating conditions of the engine 3 based on the detection signals from the aforementioned sensors 21 to 26, and carries out various control processes for controlling the engine 3 based on the results of the determination. It should be noted that in the present embodiment, the ECU 2 corresponds to EGR rate-calculating means, SOx deposition amount-calculating means, average value-calculating means, atmospheric CO2 concentration-obtaining means, error-calculating means and correction means.

Next, an exhaust gas purification process carried out by the ECU 2 for purifying exhaust gases discharged from the engine 3 will be described with reference to FIGS. 4 to 9. The present process and processes described hereinafter are executed whenever a predetermined time period dT elapses. FIG. 4 shows a process for calculating an average EGR rate rEGRavrg. In the present process, first, in a step 1 (shown as “S1” in FIG. 4; the following steps are also shown in the same way), the EGR rate rEGR at the present time point is calculated by searching a predetermined map (not shown) according to an air-fuel ratio λ and the CO2 concentration C(CO2)in in the intake air.

Next, in a step 2, an EGR gas amount. Megr is calculated using the air amount Mair and the EGR rate rEGR by the following equation (1):

Megr=Mair×rEGR/(1−rEGR)  (1)

Next, in a step 3, the integrated value of the air amount Mair (hereinafter referred to as the “air mass”) ΣMair is calculated, and in a step 4, the integrated value of the EGR gas amount Megr (hereinafter referred to as the “EGR gas mass”) ΣMegr is calculated.

Next, in a step 5, a mileage Ldrv of the vehicle V is calculated by adding the product of the vehicle speed VP and the predetermined time period dT as a control cycle of the present process (=VP×dT) to the immediately preceding value of the mileage Ldrv calculated thus far. Next, in a step 6, it is determined whether or not the calculated mileage Ldrv is not shorter than a predetermined distance Lref (e.g. 100 km). If the answer to this question is negative (NO), the present process is immediately terminated. On the other hand, if the answer to the question of the above-described step 6 is affirmative (YES), i.e. if the mileage Ldrv has reached the predetermined distance Lref, in a step 7, the average EGR rate rEGRavrg is calculated using the air mass ΣMair and EGR gas mass ΣMegr by the following equation (2):

rEGRavrg=ΣMegr/(τMair+ΣMegr)  (2)

Next, in a step 8, the air mass ΣMair, the EGR gas mass ΣMegr and the mileage Ldrv are all reset to 0, followed by terminating the present process. The average EGR rate rEGRavrg calculated as described above corresponds to the average value of the EGR rate rEGR over a predetermined time period over which the vehicle V travels the predetermined distance Lref.

FIG. 5 shows a process for calculating a shift voltage E0sft. This shift voltage E0sft represents the amount of a shift (error) of the output voltage Usens caused by the deposition of SOx on the detection electrode 22 a of the CO2 sensor 22 in the increasing direction. In the present process, first, in a step 11, it is determined whether or not it is immediately after the start of the engine 3. If the answer to this question is negative (NO), the present process is immediately terminated.

On the other hand, if the answer to the question of the above-described step 11 is affirmative (YES), i.e. if the engine 3 is immediately after the start thereof, it is judged that a low-load operation of the engine 3 is being performed and hence the EGR rate rEGR is equal to 0, so that in a step 12, a reference voltage E0base is set to a predetermined voltage Uref. This predetermined voltage Uref corresponds to the output voltage Usens from the CO2 sensor 22 in a state where the SOx deposition amount on the detection electrode 22 a of the CO2 sensor 22 is equal to 0 and at the same time the EGR rate rEGR is equal to 0. Therefore, the predetermined voltage Uref represents an atmospheric CO2 concentration C(CO2)amb (approximately 390 ppm).

Next, in a step 13, the difference between the output voltage Usens from the CO2 sensor 22 assumed at the time and the reference voltage E0base (=Usens−E0base) is calculated as the shift voltage E0sft. Then, in a step 14, an initial value SOxSNS0 of a sensor SOx deposition amount is calculated by searching a predetermined map (not shown) according to the calculated shift voltage E0sft, followed by terminating the present process. This initial value SOxSNS0 corresponds to the SOx deposition amount on the detection electrode 22 a of the CO2 sensor 22 at the start of the engine 3. In this map, the initial value SOxSNS0 is set such that it is proportional to the shift voltage E0sft.

FIG. 6 shows a process for calculating a sensor SOx deposition amount SOxSNS. This sensor SOx deposition amount SOxSNS represents the amount of SOx deposited on the detection electrode 22 a of the CO2 sensor 22 during traveling of the vehicle V after the start of the engine 3. In the present process, first, in a step 21, it is determined whether or not the present loop is immediately after termination of sensor SOx elimination control. This sensor SOx elimination control energizes the heater 22 d of the CO2 sensor 22 for a predetermined time period to thereby raise the temperature of the detection electrode 22 a to eliminate the SOx from the detection electrode 22 a.

If the answer to the question of the above-described step 21 is negative (NO), i.e. if the present loop is not immediately after termination of the sensor SOx elimination control, in a step 22, the difference between the output voltage Usens and the shift voltage E0sft calculated in the step 13 in FIG. 5 (=Usens−E0sft) is calculated as a correction voltage Ucor.

Next, in a step 23, an increased voltage Unet is calculated using the correction voltage Ucor, a predetermined coefficient αe and the EGR rate rEGR assumed at the time, by the following equation (3):

Unet=Ucor−αe×rEGR  (3)

This increased voltage Unet is obtained by subtracting the amount of an increase in the output voltage Usens (=αe×rEGR) due to an increase in the CO2 concentration C(CO2)in from the correction voltage Ucor, and represents a net increase in the output voltage Usens due to the deposition of the SOx on the detection electrode 22 a.

Next, in a step 24, the sensor SOx deposition amount SOxSNS is calculated by searching a map shown in FIG. 7 according to the increased voltage Unet, followed by terminating the present process. In this map, the sensor SOx deposition amount SOxSNS is set such that it is proportional to the increased voltage Unet.

On the other hand, if the answer to the question of the above-mentioned step 21 is affirmative (YES), i.e. if the present loop is immediately after termination of the sensor SOx elimination control, it is judged that the SOx is all eliminated from the detection electrode 22 a of the CO2 sensor 22, and the shift voltage E0sft is reset to 0 (step 25), and then the sensor SOx deposition amount SOxSNS is reset to 0 (step 26), followed by terminating the present process.

After the shift voltage E0sft is reset to 0, as described above, the correction voltage Ucor and the output voltage Usens become equal to each other, and hence in the aforementioned steps 23 and 24, the output voltage Usens is directly used as the correction voltage Ucor to thereby calculate the increased voltage Unet.

FIG. 8 shows a process for calculating a catalyst SOx deposition amount SOxLNT. This catalyst SOx deposition amount SOxLNT represents the amount of SOx deposited on the NOx catalyst 7. In the present process, first, in a step 31, it is determined whether or not the present loop is immediately after termination of catalyst SOx elimination control. This catalyst SOx elimination control supplies fuel to the upstream side of the NOx catalyst 7, thereby raising the temperature of the NOx catalyst 7 to eliminate SOx from the NOx catalyst 7. If the answer to the question of the above-described step 31 is affirmative (YES), i.e. if the present loop is immediately after termination of the catalyst SOx elimination control, in a step 32, it is determined whether or not the sensor SOx elimination control has been carried out together with the catalyst SOx elimination control.

If the answer to this question is negative (NO), in a step 33, a counter value k is incremented, followed by the process proceeding to a step 36. On the other hand, if the answer to the question of the above-mentioned step 32 is affirmative (YES), i.e. if the sensor SOx elimination control has been carried out, in a step 34, an initial value SOxSNS0 of the sensor SOx deposition amount is reset to 0, and then in a step 35, the counter value k is reset to 0.

Next, in the step 36 following the step 33 or 35, an initial value SOxLNT0 of the catalyst SOx deposition amount, referred to hereinafter, is reset to 0, and then in a step 37, the catalyst SOx deposition amount SOxLNT is reset to 0, followed by terminating the present process.

On the other hand, if the answer to the question of the above-mentioned step 31 is negative (NO), i.e. if the present loop is not immediately after termination of the catalyst SOx elimination control, in a step 38, a basic value SOxLNTbase of the catalyst SOx deposition amount is calculated using the sensor SOx deposition amount SOxSNS, the average EGR rate rEGRavrg calculated in the step 7 in FIG. 4, and a correction coefficient η sns, by the following equation (4):

SOxLNTbase=SOxSNS×ηsns×(1−rEGRavrg)/rEGRavrg  (4)

This basic value SOxLNTbase corresponds to the amount of SOx assumed to be deposited on the NOx catalyst 7 assuming that the catalyst SOx elimination control is not carried out (see FIG. 10). Further, the above-mentioned correction coefficient η sns compensates for a ratio of the area of the detection electrode 22 a of the CO2 sensor 22 to the passage area of the intake pipe 4, the difference between the BaCO3 content of the NOx catalyst 7 and that of the detection electrode 22 a, and so forth.

Next, in a step 39, the catalyst SOx deposition amount SOxLNT is calculated using the initial value SOxLNT0 of the catalyst SOx deposition amount, the basic value SOxLNTbase, a predetermined upper limit value SOxLNTrmv, and the counter value k, by the following equation (5), followed by terminating the present process.

SOxLNT=SOxLNT0+SOxLNTbase−SOxLNTrmv×k  (5)

As described hereinafter, the upper limit value SOxLNTrmv corresponds to the amount of SOx eliminated from the NOx catalyst 7 by the catalyst SOx elimination control.

FIG. 9 shows a SOx elimination control process. This SOx elimination control process performs the sensor SOx elimination control and the catalyst SOx elimination control. In the present process, first, in a step 41, it is determined whether or not the catalyst SOx deposition amount SOxLNT calculated in the step 37 in FIG. 8 is not smaller than the upper limit value SOxLNTrmv. If the answer to this question is negative (NO), the present process is immediately terminated. On the other hand, if the answer to the question of the step 41 is affirmative (YES), i.e. if SOxLNT≧SOxLNTrmv holds, it is judged that the catalyst SOx deposition amount SOxLNT is too large, and hence in a step 42, the catalyst SOx elimination control is executed. By execution of the catalyst SOx elimination control, SOx deposited on the NOx catalyst 7 is eliminated, and accordingly the catalyst SOx deposition amount SOxLNT is reset to 0 in the aforementioned step 35.

Next, in a step 43, the sum of initial value SOxSNS0 of the sensor SOx deposition amount calculated in the step 14 in FIG. 5 and the sensor SOx deposition amount SOxSNS calculated in the step 24 in FIG. 6 (=SOxSNS0+SOxSNS) is calculated as a sensor SOx total deposition amount SOxSNSgrs. Then, in a step 44, it is determined whether or not the calculated sensor SOx total deposition amount SOxSNSgrs is not smaller than a predetermined upper limit value SOxSNSrmv thereof. If the answer to this question is negative (NO), the present process is immediately terminated.

On the other hand, if the answer to the question of the above-described step 44 is affirmative (YES), i.e. if SOxSNSgrs≧SOxSNSrmv holds, it is judged that the sensor SOx total deposition amount SOxSNSgrs is too large, and hence in a step 45, the sensor SOx elimination control is executed, followed by terminating the present process. By execution of the sensor SOx elimination control, SOx deposited on the detection electrode 22 a of the CO2 sensor 22 is eliminated, and in accordance therewith, the sensor SOx deposition amount SOxSNS is reset in the aforementioned step 26.

FIG. 10 shows an example of an operation obtained by the exhaust gas purification process described heretofore. In this example, it is assumed that when a mileage L of the vehicle V is equal to L0, both the sensor SOx total deposition amount SOxSNSgrs and the catalyst SOx deposition amount SOxLNT are equal to 0. From this state, as the vehicle V travels to increase the mileage L, both the amount of SOx deposited on the detection electrode 22 a of the CO2 sensor 22 and the amount of SOx deposited on the NOx catalyst increase. The sensor SOx deposition amount SOxSNS at the time is calculated using the aforementioned FIG. 7 map, so that it increases from 0. Further, the catalyst SOx deposition amount SOxLNT is calculated using the aforementioned equation (5) (k=0), so that it increases from 0 and is equal to the basic value SOxLNTbase.

After that, when the catalyst SOx deposition amount SOxLNT reaches the upper limit value SOxLNTrmv (L=L1), the answer to the question of the step 41 in FIG. 8 becomes affirmative (YES), whereby the catalyst SOx elimination control (step 42) is executed for the first time. In accordance with the execution of the catalyst SOx elimination control, the catalyst SOx deposition amount SOxLNT is reset to 0 (step 35), and the counter value k is incremented from 0 to 1 (the step 33 in FIG. 7). On the other hand, at this time point, the sensor SOx total deposition amount SOxSNSgrs has not yet reached the upper limit value SOxSNSrmv, so that the sensor SOx elimination control is not executed, and the sensor SOx total deposition amount SOxSNSgrs continues to increase.

After that, as the vehicle V further travels, the amount of SOx deposited on the NOx catalyst increases, and the catalyst SOx deposition amount SOxLNT at the time is calculated using the equation (5) (k=1). As a consequence, the catalyst SOx deposition amount SOxLNT becomes equal to a value obtained by subtracting the upper limit value SOxLNTrmv from the basic value SOxLNTbase (=SOxLNTbase−SOxLNTrmv).

Then, when the catalyst SOx deposition amount SOxLNT reaches the upper limit value SOxLNTrmv again (L=L2), the catalyst SOx elimination control is executed for the second time, wherein the catalyst SOx deposition amount SOxLNT is reset to 0, and the counter value k is incremented from 1 to 2. On the other hand, since the sensor SOx total deposition amount SOxSNSgrs has not yet reached the upper limit value SOxSNSrmv at this time point, the sensor SOx elimination control is not executed, and the sensor SOx deposition amount SOxSNS continues to increase.

After that, as the vehicle V further travels, the amount of SOx deposited on the NOx catalyst increases, and the catalyst SOx deposition amount SOxLNT at the time is calculated using the equation (5) (k=2). As a consequence, the catalyst SOx deposition amount SOxLNT becomes equal to a value obtained by subtracting SOxLNTrmv×2 from the basic value SOxLNTbase (=SOxLNTbase SOxLNTrmv×2).

After that, when the catalyst SOx deposition amount SOxLNT reaches the upper limit value SOxLNTrmv again (L=L3), the catalyst SOx elimination control is executed for the third time, and the catalyst SOx deposition amount SOxLNT is reset to 0. Further, at this time point, the sensor SOx total deposition amount SOxSNSgrs has reached the upper limit value SOxSNSrmv, and hence the sensor SOx elimination control is executed (the step 44 in FIG. 9). In accordance with the execution of the sensor SOx elimination control, the sensor SOx deposition amount SOxSNS is reset to 0 (the step 26 in FIG. 6), and the counter value k is reset to 0 (the step 34 in FIG. 8). Thereafter, the above-described operation is repeatedly carried out.

It should be noted that when the engine 3 is stopped, the catalyst SOx deposition amount SOxLNT and the counter value k assumed at the time are stored, and are used as the initial values SOxLNT0 and k at the following start of the engine 3, respectively.

As described heretofore, according to the present embodiment, since the detection electrode 22 a of the CO2 sensor 22 contains BaCO3, it is possible to detect not only the CO2 concentration C(CO2)in in the intake air but also the sensor SOx deposition amount SOxSNS. Further, the catalyst SOx deposition amount SOxLNT is calculated according to the EGR rate rEGR and the sensor SOx deposition amount SOxSNS, based on the fact that the relationship between the sensor SOx deposition amount SOxSNS and the catalyst SOx deposition amount SOxLNT is determined according to the EGR rate rEGR, which makes it possible to accurately calculate the catalyst SOX deposition amount SOxLNT. Further, it is possible to perform the catalyst SOx elimination control while supplying a just enough amount of reducing agents according to the catalyst SOx deposition amount SOxLNT calculated as above, whereby it is possible to improve the fuel economy of the engine 3 and reduce exhaust emissions therefrom.

Further, the average EGR rate rEGRavrg of the vehicle V travelling the predetermined distance Lref is calculated for use in calculating the catalyst SOx deposition amount SOxLNT, so that even when the EGR rate rEGR=0 holds at a time point, it is possible to calculate the catalyst SOx deposition amount SOxLNT.

Further, at the start of the engine 3, the shift voltage E0sft is calculated, and the sensor SOx deposition amount SOxSNS is calculated using the shift voltage E0sft, and hence even when SOx has already been deposited on the detection electrode 22 a of the CO2 sensor 22 at the start of the engine 3, it is possible to accurately calculate the catalyst SOx deposition amount SOxLNT while compensating for the adverse influence of the SOx.

It should be noted that the present invention is by no means limited to the embodiment described above, but can be practiced in various forms. For example, although in the above-described embodiment, the average EGR rate rEGRavrg is calculated whenever the vehicle travels the predetermined distance Lref which is fixed, this predetermined distance Lref is not required to be fixed. For example, the predetermined distance Lref may be set to a value which becomes smaller as the catalyst SOx deposition amount SOxLNT becomes closer to the upper limit value SOxLNTrmv. This makes it possible to calculate the average EGR rate rEGRavrg in a more fine-grained manner as the catalyst SOx deposition amount SOxLNT becomes closer to the upper limit value SOxLNTrmv, thereby making it possible to further enhance the calculation accuracy of the catalyst SOx deposition amount SOxLNT to carry out the catalyst SOx elimination control in more appropriate timing. Further, the average EGR rate rEGRavrg may be calculated whenever a predetermined time period elapses, not whenever the vehicle travels a predetermined distance.

Further, although in the above-described embodiment, the shift voltage E0sft is calculated only once immediately after the start of the engine 3, it is only required to calculate the shift voltage E0sft when the EGR rate rEGR=0 holds. For example, instead of the steps 25 and 26 in FIG. 6 which are carried out immediately after termination of the sensor SOx elimination control, the following process may be carried out.

First, immediately after termination of the sensor SOx elimination control, the EGR operation is stopped by fully closing the EGR control valve 6 b. Next, after a predetermined time period elapses from the stoppage of the EGR operation, it is judged that the EGR rate rEGR is equal to 0, and the shift voltage E0sft and the initial value SOxSNS0 are calculated in a manner similar to the above-described process in FIG. 5. Then, the subsequent sensor SOx deposition amount SOxSNS is calculated using the calculated shift voltage E0sft and the calculated initial value SOxSNS0. In this case, the shift voltage E0sft is also calculated during traveling of the vehicle V, whereby even when SOx is not completely eliminated from the detection electrode 22 a by the sensor SOx elimination control, it is possible to maintain the calculation accuracy of the catalyst SOx deposition amount SOxLNT.

Further, although in the above-described embodiment, a predetermined value corresponding to a normal atmospheric CO2 concentration is used as the predetermined voltage Uref representative of the atmospheric CO2 concentration C(CO2)amb, an atmospheric CO2 sensor for detecting the atmospheric CO2 concentration C(CO2)amb may be provided for using a detection value detected by the atmospheric CO2 sensor.

Further, although in the above-described embodiment, the present invention is applied to the lean burn gasoline engine installed on an automotive vehicle by way of example, this is not limitative, but it can be applied to various types of engines, such as diesel engines other than gasoline engines, and engines for other than automotive vehicles, such as engines for ship propulsion machines, e.g. an outboard motor having a vertically-disposed crankshaft. Further, it is possible to change details of the construction of the embodiment within the spirit and scope of the present invention.

INDUSTRIAL APPLICABILITY

As described hereinabove, the exhaust gas purifying system according to the present invention is capable of accurately calculating the amount of SOx deposited on the NOx catalyst, and is useful in improving the fuel economy of the engine and reducing exhaust emissions therefrom.

REFERENCE SIGNS LIST

-   1 exhaust gas purifying system -   2 ECU (EGR rate-calculating means, SOx deposition amount-calculating     means, average value-calculating means, atmospheric CO2     concentration-obtaining means, error-calculating means, correction     means) -   3 engine (internal combustion engine) -   4 intake pipe (intake passage) -   4 a confluent portion -   5 exhaust pipe (exhaust passage) -   6 a EGR passage (EGR pipe) -   7 NOx catalyst -   22 CO2 sensor -   22 a detection electrode -   C(CO2)in CO2 concentration in intake air -   rEGR EGR rate -   SOxSNS sensor SOx deposition amount (detection value detected by the     CO2 sensor) -   SOxLNT catalyst SOx deposition amount (amount of SOx deposited on     the NOx catalyst) -   Lref/dT time elapsed after the start of the engine until mileage     reaches a predetermined distance (predetermined time period) -   rEGRavrg average EGR rate (average value of the EGR rate over a     predetermined time period.) -   C(CO2)amb atmospheric CO2 concentration 

1. An exhaust gas purifying system for an internal combustion engine, for purifying exhaust gases discharged from the engine to an exhaust passage, comprising: a NOx catalyst disposed in the exhaust passage, for purifying NOx in exhaust gases; an EGR passage that branches from the exhaust passage on an upstream side of said NOx catalyst and joins an intake passage, for recirculating part of exhaust gases discharged into the exhaust passage into the intake passage; a CO2 sensor that is disposed in the intake passage at a location downstream of a confluent portion where said EGR passage joins the intake passage, and includes a detection electrode containing BaCO3, for detecting a CO2 concentration in intake air sucked into the engine; EGR rate-calculating means for calculating a recirculation rate of exhaust gases recirculated via said EGR passage as an EGR rate; and SOx deposition amount-calculating means for calculating an amount of SOx deposited on said NOx catalyst as a SOx deposition amount, according to the calculated EGR rate and a detection value detected by said CO2 sensor.
 2. The exhaust gas purifying system as claimed in claim 1, further comprising average value-calculating means for calculating an average value of the EGR rate over a predetermined time period, and wherein said SOx deposition amount-calculating means calculates the SOx deposition amount using the calculated average value of the EGR rate.
 3. The exhaust gas purifying system as claimed in claim 1, further comprising atmospheric CO2 concentration-obtaining means for obtaining an atmospheric CO2 concentration; error-calculating means for calculating an error of the detection value detected by said CO2 sensor, the error being caused by a deposition of SOx on said detection electrode of said CO2 sensor, according to the CO2 concentration detected by said CO2 sensor when no exhaust gases are being recirculated via said EGR passage, and the obtained atmospheric CO2 concentration; and correction means for correcting the detection value detected by said CO2 sensor according to the calculated error. 